1. Field of the Invention
The present invention generally relates to throttle control in an internal combustion engine. More specifically, it relates to throttle system and method enabling reduction of air-flow noise occurring in response to changes in the amount of the throttle opening.
2. Description of the Related Art
As is widely known, the quantity of air supplied to an internal combustion engine (will be referred to as “engine” where appropriate) is adjusted at a throttle valve provided in an intake passage of the engine. When the throttle valve quickly opens, it disturbs the flow of air so that the air starts swirling. Such swirl airflow may sometime cause specific noise (will be refereed to as “throttle inflow noise”). Meanwhile, an intake manifold forms one portion of the intake passage, and it is known that a resin-made intake manifold is now increasingly used. Such a resin intake manifold, however, inevitably allows the above-stated throttle inflow noise to be easily propagated or emit to the outside.
To counter this, for example, a known method provides a net or plate in the intake passage to regulate the flow of air downstream of the throttle valve and thereby reduce throttle inflow noise. However, this method involves drawbacks of reduced engine output due to increased air resistance within the intake passage, increased weight, increased production cost, etc. Also, this method is not effective enough to reduce the throttle inflow noise when a resin made intake manifold is used for the reason already mentioned.
In view of the above, another method has been proposed which reduces throttle inflow noise by limiting a rate at which the throttle valve opens (will be referred to as “throttle opening rate”) below a specific limit rate under given conditions so that the throttle valve opens relatively slowly (see Japanese Laid-opened Patent Application No. 2001-98958, No. 2001-234758). For example, Japanese Laid-opened Patent Application No. 2001-98958 proposes opening the throttle valve at a lower rate when the throttle angle is smaller than a specific value.
Although it is true that the throttle inflow noise can be reduced by opening the throttle valve at a reduced rate as described above, lowering the throttle opening rate may deteriorate the controllability of the engine output.